Problem: There were 21 people riding on a train before it stopped and some number of people got off. Now there are 13 people riding on the train. How many people got off the train at the stop?
Answer: The number of people who got off is the difference between the number who were riding before and the number who are riding now. The difference is $21 - 13$ people. $21 - 13 = 8$ people got off.